This invention relates generally to a method of forming an improved bearing surface and more particularly to a method of forming the bearing surface of a sealing member, such as a piston ring.
A requirement of the bearing surface of a dynamic sealing member, such as a piston, is that it mate perfectly with the surface against which it seals. In addition, the bearing surface of a dynamic sealing member must be wear resistant and able to withstand high temperatures.
Hard chromium plate has been found to be satisfactory as the bearing surface of a dynamic sealing member, but its application has several inherent disadvantages. For economic reasons it is necessary to electrodeposit the hard chromium at relatively high deposition rates which results in large nodules and a coarse grain structure. To prevent scoring and scuffing (i.e., localized welding) between the sealing member and the surface being sealed, it is necessary to lap the bearing surface to wear down the protruding nodules formed during plating and to remove other surface irregularities that result from plating over an imperfect substrate. The required lapping operations are necessarily expensive.
A second disadvantage of hard chromium plate as the bearing surface of a sealing member results from the fact that the bearing surface must initially wear until it is in perfect conformity with the surface to be sealed. The wear resistant property of hard chromium plate thus become a disadvantage in that it extends the period required to reach this conformity (i.e., the break-in period) and hence, extends the period during which the sealing member performs at less than designed efficiency.
These disadvantages are well-known and numerous attempts have been made to overcome them. The proposed solutions have generally centered around overplating the hard chromium with a softer, less wear resistant metallic material. These solutions have their own disadvantages in that the softer material wears away too rapidly or sloughs off. Also, because of its softness it is subject to localized welding with the surface being sealed. Etching of the hard chromium has also been proposed to make it less wear resistant. This has been unsuccessful because it re-exposes the coarse nodules that were originally removed by lapping and which cause scratching and scuffing.